'It's gonna be terrific,' gushed Mel Brooks, seeing the sceptical cock of my eyebrow as he mentioned the movie. We were taking tea at the Savoy in February 2004 with the London run of The Producers still months ahead and casting in all kinds of chaos. What comes round comes round, but making a film of the musical of the film about a musical struck me as being some way beyond a joke.

Some hopes. The third coming of the first musical in history about double book-keeping, Swedish leg exercises and the little-known Busby Berkeley routines of Adolf Elizabeth Hitler falls flatter than a Jewish joke in the Vatican as the show is transferred to screen with no added gags or scenes - nothing but an extra squeeze of the public lemon until the pips squeak and, this time, the joke is on us. The biggest display ads of the season could not disguise the vacuity of the event.

Laugh? I couldn't raise a wince. As a card-carrying cheerleader of The Producers since the cult film of 1968, this movie was over for me with the overture, a groaning of major chords that in the theatre elicit anticipatory chortles all around the house and in the cinema was received with all the hilarity of a paper handkerchief. Max Bialystok himself could not have produced a bigger flop.

As I sank lower and lower in seat and spirits, the only cheer to be had was from the familiar recognition that, for a critic, bad art is good for you in the sense that it reveals more about the qualities of genius than genius itself. Read the novels of Heinrich Mann to understand the grandeur of Thomas. Hear a symphony by Bax to appreciate the superiority of Vaughan Williams. Watch Lorin Maazel's opera 1984 at Covent Garden to grasp how a great conductor's musical knowledge and superb technique fall short of creative originality.

The Producers has a litany of things that are wrong with it as long as Elton John's wedding guest list. But the faults are instructive in the ways that they demonstrate why theatre has survived the onslaught of cinema and TV and why movies can be turned into great musicals, but seldom the other way round.

The flaws flap around the film like munchkins in the Wizard of Oz. Susan Stroman, ingenious Broadway choreographer that she is, has never directed behind a camera, and it shows. She has no eye for exteriors, panoramic movement and peripheral flutter, all the little things that catch the eye in a movie's flat moments. The stage is her set, and that's it. Where West Side Story, South Pacific and the Sound of Music ö perhaps the only musicals that converted triumphantly to film - gained a dimension of realism from being shot under God's own canopy, The Producers stay agoraphobically indoors. Few tourists escape Salzburg these days without being offered The Sound of Music day trip. Nobody will ever make a pilgrimage to The Producers' backlot.

The cast are hapless victims of genre constriction. Nathan Lane, who fills a stage with terrifying bonhomie, loses his menace and much of his presence on screen; he seems to be one of those actors, like Antony Sher, whose qualities of excess are shrunk by the viewfinder. Matthew Broderick and Uma Thurman have magic but no new moves. The callisthenics that looked so sensational in the West End are uninvolving in the cinema where anything can be faked.

The script by Brooks and Thomas Meehan (who was jointly responsible for Maazel's opera) sticks religiously to the original, as if in hope that dressed-up devotees will chant along with the best lines, just as they do in Singalong The Sound of Music, that other holiday gay romp at Berchtesgaden. The hand of Mel, once so fleet and sure, lies heavy on the enterprise. It is no secret that he demands click-heel adherence to the one true way of playing the central roles. Henry Goodman, who dared to deviate on Broadway, was summarily cashiered; Richard Dreyfus, who deviated in London, was on a plane home before the curtain went up.

Any author is entitled to love his work, but when a show falls in love with itself as this one has done, savouring its stale emanations as if they were perfumed, the audience will feel excluded, if not abused. The appeal of The Producers as a musical was its wicked wish to mock the whole pantomime of theatrical production and all who play in it. In the theatre, actors and audience are in it together, sharing the show and simultaneously standing back from it.

In the cinema, the dimensions of depth and detachment disappear and, with them, the possibility of irony. It takes two to self-parody, and it cannot be done on a flat wall. Theatre is an act of communication, cinema a means of mass exploitation. One is about dialogue, the other propaganda.

The sole heartening aspect of The Producers as a movie is its like-for-like vindication of live art over canned, its proof positive ö and how positive ö that the musical will carry on running on Broadway, in the West End and on tour for years after its synthetic transposition is consigned to the dump bins of DVD stores and the lower recesses of cable television.

These truths do not bode well for musicals that lied ahead. Rent, a Broadway hit that never travelled, is due on screens next year and Elton John and Stephen Daldry will doubtless receive flattering offers to licence the next turn of Billy Elliot as Mel Brooks did with The Producers. They must resist the temptation.

The ultimate lesson of The Producers, as any stand-up confirm will confirm, is that a joke is like milk in tea. Fresh, cold and in the right dollop, it suffuses you with comfort and warmth. Long-life, or a drop too many, and it puckers the lips and sours the mood. There is scant joy to be derived from a joke that has grown whiskers. Let's mark it down to experience. The Producers movie may save us from worse.

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